Acoustic guitar feedback naturally occurs when output from an amplifier or speaker excites the strings of a guitar. This then creates a signal to send to the speakers, thereby creating an additive loop. Guitar feedback is different from standard microphone acoustic feedback, because the guitar strings excite in such a way as to keep feedback at the resonant frequency of the guitar string (or a harmonic of that frequency). In some cases, the guitar body and/or pickup may begin to resonate, but such feedback is not usually musical or desirable. Guitar feedback can be manipulated by guitar players in a musical manner, and is therefore considered by some players to be desirable.
A limiting factor in the creation of natural guitar feedback is that extreme output levels are typically required from the amplifier in order for the sound waves to have enough energy to sufficiently excite the guitar strings. Moreover, if guitar feedback is successfully obtained, it can be quite hard to get feedback to occur at the desired harmonic of the note being played.